An ignition device for an internal combustion engine supplying electrical energy from two ignition coils to a single spark plug is known (refer to Patent Document 1, for example). According to this technology, there is provided a first switching unit for turning on/off a power supply to a primary coil, which is one of the ignition coils, from an on-vehicle battery, and a second switching unit for turning on/off the power supply to another primary coil, which is another one of the ignition coils, from the on-vehicle battery. Then, so-called on/off switching based on a full transistor system of the first and second switching units is repeated alternately so that on-periods of the first and second switching units do not overlap. Thus, a stable discharge state with no interrupted discharge can be maintained.
However, according to the technique of Patent Document 1, when the On-periods of the first and second switching units are overlapped, a discharge current decreases rapidly and a spark discharge cannot be maintained, and conversely, when Off-periods of the first and second switching units are overlapped, discharge timings overlap, thus the discharge current increases rapidly causing significant damage to an electrode in the spark plug. Therefore, since in order for the on/off state of the first and second switching units to follow variation in individual parts or a change of spark timing due to differences in operating states of the internal combustion engine, it is necessary to perform the on/off switching with high accuracy, construction of control logic of the technology in the Patent Document 1 becomes complicated, and a calculation load in actual operation is considered to be large.